MODBUS Device Interface Segregation


Thread Starter

Matthew Raterman

Is there any set of rules/guidance/best practices for seperating Modbus devices that are communicating to a single Master? I have project were there are several Modbus TCP/IP devices that are connected to the host DCS through a modbus TCP card. This card can handle up to 256 devices, but due to concern of failure, etc., it is preferred to separate these device out onto multiple cards. I'm looking for suggestions/best practices that could be used as a design guideline for determining what devices will go together on what card based on controlled equipment, device type, manufacturer, interface criticallity, etc.
First of all MODBUS shall not be considered as a backbone of data transfer between Lower level devices and the master which is DCS for control applications, where as it can be used for monitoring purpose/ Non critical equipment like Ordinary pump start/stops/Local/Remote selection,Analyzer/Flowemeter interfaces.

1. Number of frames (packets) shall be minimized.

2.Size of Analog Signal packets/frame shall be minimized in order to give room for the lower level devices to respond within the MODBUS Frame time out. This will minimize the heavy traffic on MODBUS n/w.

3. Minimize the number of protocol converters in between the MODBUS Slave devices and DCS. If it's of PLCs try to avoid n/w switches in between to avoid the inherent latency.

4. Response time out/Number of retries up on response time out/interval of connection retries like these settings shall be adjusted at DCS based on the data update response from the MODBUS SLAVES.